An allele is simply a version of a gene. Just as a shoe comes in different sizes, a single gene can come in different forms, and each form is called an allele. You inherit two alleles for every gene, one from each parent, and the combination you end up with shapes traits like your eye color, blood type, and susceptibility to certain diseases.
How Alleles Sit on Your Chromosomes
Your DNA is organized into 23 pairs of chromosomes. Each gene occupies a specific spot on a chromosome, called a locus. Because chromosomes come in pairs, you have two copies of every gene, one on each chromosome in the pair. Those two copies might be identical, or they might be different versions. Each version is an allele.
Think of it like an address. The locus is the street address, and the allele is which tenant lives there. At the same address on chromosome 9, for example, one person might carry the A allele for blood type while another carries the B allele. The address is the same; the occupant differs.
Dominant and Recessive Alleles
When your two alleles for a gene are different, only one may actually show up as a visible trait. The allele that gets expressed is called dominant, and the one that stays hidden is called recessive. If you carry one allele for brown eyes (dominant) and one for blue eyes (recessive), your eyes will be brown. The blue-eye allele is still in your DNA, and you can still pass it to your children, but it doesn’t influence your own eye color.
For a recessive trait to appear, you need two copies of the recessive allele. This is why two brown-eyed parents can have a blue-eyed child: if both parents carry one hidden blue-eye allele, their child has a chance of inheriting both copies.
Homozygous vs. Heterozygous
Geneticists use two terms to describe your allele combinations. If your two alleles for a gene are identical, you’re homozygous for that gene. If they’re different, you’re heterozygous. These terms come up often in genetic testing results and health contexts. A person who is heterozygous for a disease-related gene carries one normal allele and one altered allele, which can have very different health implications than carrying two altered copies.
When Alleles Don’t Follow Simple Rules
Not every gene follows the straightforward dominant-beats-recessive pattern. There are several other ways alleles can interact.
Incomplete dominance happens when neither allele fully overpowers the other, producing a blended trait. Snapdragon flowers are a classic example: crossing a red-flowered plant with a white-flowered plant produces pink offspring. The result is an intermediate that neither parent displayed.
Codominance occurs when both alleles are fully expressed at the same time, with no blending. The best example is the ABO blood group system. The gene for blood type has three alleles: A, B, and O. The A and B alleles each produce a distinct marker on the surface of red blood cells, while the O allele produces no functional marker at all. If you inherit one A allele and one B allele, both markers appear on your cells, giving you type AB blood. Neither allele masks the other.
More Than Two Alleles Can Exist
Although you personally carry only two alleles for any gene, the broader population can have many more versions circulating. Blood type is again a good illustration: three alleles (A, B, and O) exist across the human population, but any single person only has two of them. Some genes have dozens or even hundreds of known alleles. The more alleles that exist for a gene, the greater the diversity of traits you see in a population.
Alleles and Evolution
Alleles are the raw material of evolution. When a population’s mix of alleles shifts over generations, evolution is happening. Three main forces drive that shift.
Natural selection favors alleles that help individuals survive and reproduce. Over time, beneficial alleles become more common. Genetic drift is random change, especially powerful in small populations where chance events (a storm killing a group of animals, for instance) can eliminate certain alleles entirely. Gene flow happens when individuals migrate between populations, carrying their alleles with them and reshuffling the genetic mix.
These forces work together. A small, isolated population might lose an allele through drift that would have been preserved by natural selection in a larger group.
Sickle Cell: An Allele With Two Faces
One of the most striking real-world examples of alleles in action involves sickle cell disease. A single change in the gene for hemoglobin, the protein that carries oxygen in red blood cells, creates an allele called HbS. If you inherit two copies of HbS (homozygous), your red blood cells can deform into a rigid sickle shape, causing serious health problems. Most people with two copies historically did not survive to adulthood without treatment.
But carrying just one copy (heterozygous) tells a completely different story. People with one normal allele and one HbS allele are largely healthy, and they gain significant protection against malaria. The malaria parasite has difficulty thriving in red blood cells that contain the HbS protein, and carriers are far less likely to develop the most dangerous form of the disease, cerebral malaria.
This trade-off explains why the HbS allele is common in regions where malaria is widespread, particularly across sub-Saharan Africa and parts of the Mediterranean. Natural selection maintains both alleles in the population: the normal allele avoids sickle cell disease, while the HbS allele protects against malaria. Geneticists call this balanced polymorphism, where two competing pressures keep both alleles in play rather than one replacing the other.
Alleles in Modern Genetic Testing
When you get results from a genetic test or a consumer DNA kit, the data is built on allele information. Most of the genetic variation between people comes down to spots in the DNA where a single “letter” of the genetic code differs. These spots are called SNPs (pronounced “snips”), and each SNP has alleles, typically just two. More than 95% of these variable spots in the human genome are biallelic, meaning only two versions exist across the population.
At each SNP, one allele is more common (the major allele) and one is rarer (the minor allele). When researchers link a SNP to disease risk, they identify which allele raises that risk. Sometimes it’s the rare version, sometimes the common one. Your genetic test results are essentially a readout of which alleles you carry at thousands or millions of these positions, compared against what researchers have learned about each one.
Understanding alleles gives you the foundation for making sense of inheritance patterns, genetic test results, and why siblings from the same parents can look and feel so different. Every trait influenced by your DNA comes back to which combination of alleles you happened to receive.